A drag force acts on an object which moves in a fluid environment such as air or water. This drag force includes several specific drag forces wherein the main one is known as a pressure drag force. The pressure drag force is caused by a net pressure force acting on the object. The rear end contribution to the pressure drag is called "base drag". Flow separation at the base of the moving object creates a vortex system and reduces base pressure thus increasing drag. This problem exists for truncated objects, which have blunt bases, such as a box, a cylinder and the like.
Reference is now made to FIG. 1A which is a schematic illustration of a device for reducing drag which is known in the art (Frey, D. "Guide Vores" Foschung Ing Wessen, 1933 and Hoemer, S. F. "Fluid Dynamic Drag", 1958 p. 3-27). One of the ways known in the art for reducing the base vortex strength in two-dimensional objects such as high aspect ratio wings, is by utilizing winglets near the base of the wing or behind it. wing 10 includes four winglets 12, 14, 16 and 18, which reduce the base drag by depressing the ascilatory vortex shedding from the base.
The asymmetric, ascilatory vortex shedding which greatly increases the base drag in a 2 dimensional configuration does not exist in three dimensional bodies.
Reference is now made to FIG. 1B which is a schematic illustration of a device, known in the art (Maull, D. J. "Mechanisms of Two and Three Dimensional Base Drag", Plenum Press, 1978), which was tested for aerodynamic drag reduction. A three dimensional blunt object 20, which in the present example is a truck, includes two rear side flow deflectors 22 and 24 and a rear top deflector 26. This configuration has proved to be inefficient in reducing the base drag and has even shown slight increases in the drag force, as compared to the baseline configuration of a truck without such deflectors.
Another device aiming at base drag reduction on blunt-based trailers is described in U. S. Pat. No. 5,348,366 (Baker and Levitt, 1994). It is shown in FIG. 3 (of Baker). The amount of drag reduction achieved by deploying the device shown in FIG. 3 is 15%. The mechanism of drag reduction is similar to that in boattailing a blunt axi-symmetric object and thus increasing its base pressure, as was suggested by Mair (1965).
Other devices for reducing the base drag of airborne axi-symmetric bodies use air bleed through the blunt base (U.S. Pat. No. 4,807,535 by M. Schilling and M. Reuche (1989) and U.S. Pat. No. 4,554,872 by U. Schleicher (1985)). These devices require, however, modification of the internal volume to accommodate the charge used to accommodate the base bleed jet.